"And G-d said to Moshe and Aharon, 'because you did not believe in me, to sanctify me in the eyes of the Children of Israel, for this reason you shall not bring this congregation to the Land which I have given to them... Aharon will be gathered unto his people, because you defied My word at Mei Merivah.'" [20:12, 24]
These words should strike us as not merely puzzling, but bizarre - it's preposterous! Moshe, who ascended Har Sinai, spoke "face to face" (as it were) with HaShem Himself, did not _believe_ in Him? Aharon, who directed the Services in the Tabernacle, and caused the Divine Presence to descend upon the camp, did not believe in G-d?
If someone were to read only this passage, he might think that HaShem was accusing Moshe and Aharon of atheism or idolatry. Yet, obviously, this is not so. In fact, the error of Moshe and Aharon was so subtle that we perhaps cannot understand it - the Torah never specifies precisely what they did wrong, and many of the commentators present various possibilities.
Our Sages say that HaShem "is as precise as a hair's breadth" with those who are close to Him. Moshe and Aharon came so close to G-d that for them a "sin" was something so small as to be beyond our perception, no more comprehensible to our minds than microbes are visible to our eyes.
On Rosh HaShanah, there is a tradition to go to a body of water and "cast off" one's sins, as it were, and ask that they be covered over like water covers and hides the fish who swim within it. Many Chassidim have a custom of to take bread crumbs along and throw them in, to give physical expression to this idea.
There is a story which says that a particular Chassidic Rebbe threw crumbs into the water, at which point one of his Chassidim bounded into the lake and began to retrieve them. When questioned, the Chassid explained: "what to the Rebbe are sins, are Mitzvos where I'm concerned!"
For years, I couldn't understand this story or its intended lesson. A transgression is a transgression! But then, I heard that the Chofetz Chaim, Rabbi Yisrael Mayer HaKohein Kagan (perhaps the greatest known Torah scholar of this century) once repented on Yom Kippur for having wasted eight minutes from Torah study during the previous year.
Can we imagine wasting merely eight minutes in an entire year? I would be extremely happy to say that I had managed to waste no more than eight minutes on a given afternoon! Maybe, maybe I've spent a few hours without wasting eight minutes. Maybe.
It's impossible to imagine being able to account for every moment of every day, save eight minutes (I would almost be relieved to be told that I had heard this story incorrectly). And this is what the Chassid was saying: for us, it would be a great Mitzvah! The sins of great people occur at such a level of precision, that _reaching_ that level, to be worthy of being judged at that level, would be a phenomenal achievement.
The Torah itself, and the books of the Prophets as well, are replete with similar stories of "grave sins" committed by our forebears - even as we are told that these same individuals were extremely holy and pious, so close to G-d as to receive Divine Prophecy. Just like Moshe and Aharon at Mei Merivah, these stories are obviously not fictitious, but neither can they be read "at face value" without comprehension - any more than we can refer to a literal hand, arm, or face of G-d.
Precisely because the Bible is dealing with individuals on an exalted spiritual level, if it were to tell us merely what they did, we would be unable to perceive anything wrong. For those people, their behavior was no less a transgression than if a more common individual had committed a great crime like murder, adultery or idolatry - and thus the Prophets use severe language, similar to HaShem's own words that Moshe and Aharon "did not believe" in Him. Just like the anthropomorphic references to HaShem, these passages use language which we can understand, so that we can learn from them.
Every human being is just that - human - and no one is perfect. Even as we are humbled by recognition of the heights reached by prophets and great scholars, we should never lose hope, or imagine that those who came close to G-d were truly angels, without inner struggles or difficulties. This is the lesson the Torah brings home to us when attributing unimaginable 'sins' to our forebears. And yet it is also incumbent upon us to realize that we could be, ourselves, so close to HaShem that our 'sins' would be something we could not even recognize today.